1. Field of the Invention
The present invention relates to a mechanism and a method for mechanically removing a substrate clamped to a base and, more particularly, to a mechanism and a method for mechanically removing a substrate which is clamped to an electrode in a plasma processing apparatus through a residual static charge.
2. Description of the Related Art
A well-known electrostatic clamp for holding a substrate, such as a semiconductor substrate to an electrode in a plasma processing apparatus is shown in FIG. 8. The plasma processing apparatus of FIG. 8 is used, for example, for dry-etching of semiconductor substrates. A disk-shaped substrate 101 is placed within the plasma processing apparatus on a pedestal 103. The pedestal 103 is positioned on an electrode 104. Substrate-elevating pins 102 are disposed within the electrode 104 and below the substrate. The pins 102 are lifted so that they extend through openings in the pedestal to elevate the substrate 101. The plasma processing apparatus of FIG. 8 also includes a processing chamber 105 surrounding the electrode 104, a prop 106 for vertically moving the electrode 104, and a frame 107. A disc-shaped cover 125 is mounted on the top surface of the electrode 104 surrounding the pedestal 103.
The substrate 101 is clamped to the electrode 104 by an electrostatic clamping force which is created by a static charge accumulated on the dielectric material. In order to remove the substrate 101 from the pedestal 103, the electrode 104 is lowered together with the prop 106 along a shaft 106a. As the electrode is lowered, a stopper 108 which is mounted at the lower end of the shaft 106a, engages the lower wall of the frame 107. When the electrode 104 is moved further downward, the pins 102 protrude beyond the top surface of the pedestal 103. The substrate 101 is thereby separated from the pedestal 103 by the pins 102.
In the mechanism for removing the substrate shown in FIG. 8, the substrate 101 is slightly raised by the pins 102, when those portions of the substrate 101 which are lifted by the pins 102 are slightly separated from the pedestal 103. However, those portions of the substrate 101 which are not lifted by the pins 102, especially the edges of the substrate, still adhere to the pedestal 103 because of a residual static charge remaining on the pedestal 103. If the substrate 101 is raised further by the pins 102, a part of the edge of the substrate 101 remains adhered to the pedestal 103, as shown in FIG. 9 causing the substrate to tilt. When the pins 102 stop moving upward so that the substrate 101 may be removed from the processing chamber 105, then the substrate 101 remains in a tilted position. In this tilted posture, when a hand 30 of a transferring robot is inserted between the substrate 101 and the pedestal 103 to remove the substrate, the substrate is not accommodated within the proper region on the hand 30 between the four claws 31 and indicated by the broken line. This problem hinders the transfer of the substrate from the processing chamber. However, if the substrate 101 was maintained in a horizontal posture during the removal of the substrate from the processing chamber, then the problem associated with the prior art would be eliminated.